Philae

Description

Philae is the landing craft of the Rosetta mission, designed to touch down on Comet 67P/Churyumov-Gerasimenko in 2014. The scientific objectives are to determine the physical properties of the comet's surface and subsurface and their chemical, mineralogical and isotopic composition. This information will be used in tandem with the data returned by the Rosetta orbiter to characterize the comet.

Spacecraft and Subsystems

The Philae spacecraft is a partial hexagonal cylinder, approximately 1 meter across and 80 cm high, open on one end, supported on a long squat tripod and consists of a baseplate, experiment platform and hood. The structure is made of high modulus carbonfiber with an aluminum coating in a polygonal sandwich construction. The landing gear consists of a central telescopic tube connecting lift and torque mechanism located in the cavity of the lander's body connected at the lower end by a kardanic joint to the center of the tripod. The three lander legs are equipped with shock absorbers to inhibit bouncing in the low gravity. Push-down and hold-down thrusters are used to accelerate descent and impede rebound after touchdown. A harpoon connected to a tether will be fired into the surface of the comet to anchor the lander. Power will be provided by low intensity, low temperature GaAs solar cells mounted on the top panel of the lander hood and a 970 Whr and 110 Whr battery. The lander will communicate with the Rosetta spacecraft via a 1 W S-band transmitter. A flywheel provides 1-axis stabilization during the descent. Total mass of the lander is about 100 kg. Philae will be carried on the side of the Rosetta orbiter until it reaches the comet.

The Philae surface science package, with a total mass of about 21 kg, includes an alpha-proton-X-ray spectrometer (APXS) to determine elemental composition; two gas chromatograph/mass spectrometers: the Cometary Sampling and Composition Experiment (COSAC) and Methods Of Determining and Understanding Light elements from Unequivocal Stable isotope compositions (MODULUS/Ptolemy) to study composition, isotopic abundances and to identify complex organic molecules in cometary material; Surface Electrical, Seismic, and Acoustic Monitoring Experiments (SESAME) to investigate surface material acoustically, measure dielectric properties of the environment, and monitor dust impacts; Multi-Purpose Sensors for Surface and Subsurface Science (MUPUS) to study physical properties of the comet; Comet Nucleus Sounding Experiment By Radiowave Transmission (CONSERT) to investigate electrical characteristics of the nucleus bulk material and internal structure; Rosetta Lander Magnetic field investigation and Plasma monitor (ROMAP) to investigate the comet's magnetic field and interaction with the solar wind; in-situ imaging systems known as Comet Nucleus Infrared and Visible Analyser (CIVA) and the Rosetta Lander Imaging System (ROLIS), and a drill and sample collector (SD2).

Mission Profile

Rosetta and Philae were launched at 7:17 UT on 2 March 2004 on an Ariane 5 G+ from Kourou, French Guiana and will rendezvous with Comet Churyumov-Gerasimenko in mid-2014. The spacecraft will enter a heliocentric drift phase to intercept the comet at a point close enough to allow communication with the Earth in 2014. More details on the journey to the comet can be found in the Rosetta mission description at:

http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=2004-006A

On Rosetta's arrival at the comet and insertion into orbit, a suitable landing site will be chosen based on the images sent back to Earth. In November 2014, on confirmation that the orbiter is aligned correctly, Philae will be commanded to self-eject, unfold its three legs, and descend towards the surface from an altitude of roughly 1 km. The lander will touch down at less than 1 meter/sec, and the legs and thrusters will prevent the lander from bouncing. The legs can rotate, lift or tilt to return Philae to an upright position. Immediately after touchdown, a harpoon will be fired to anchor Philae to the ground and prevent it escaping from the comet's extremely weak gravity. After touchdown the lander will deploy its instruments. The minimum mission target is one week of operation on the surface, but operations of many months are possible.

Philae is named after the island in the river Nile on which an obelisk was found that had a bilingual inscription including the names of Cleopatra and Ptolemy in Egyptian hieroglyphs. This provided the French historian Jean-Francois Champollion with the final clues that enabled him to decipher the hieroglyphs of the Rosetta Stone and unlock the secrets of the civilisation of ancient Egypt.